We have recently identified a serum protein, Acrp30 (adipocyte complement-related protein of 30kDa), a secretory protein expressed exclusively in adipose tissue. Our structural studies established Acrp30 as the founding member of a new branch of the Tumor Necrosis Factor superfamily. Acrp30 levels are regulated by insulin both trancriptionally as well as at the level of secretion from adipocytes. However, circulating Acrp30 levels are not affected by total adipose mass per se. Acrp30 circulates in serum as an active complex at a concentration of 2-20nM, binds specifically to hepatocytes and may also have functional binding sites on muscle cells. Our hypothesis states that Acrp30 is an adipocyte-specific factor that governs various aspects of whole body energy homeostasis. Its mode of action may resemble that of its structural homolog TNFalpha, interfering with the insulin signal transduction cascade in peripheral tissues, such as liver and muscle. We propose to study the biological role of this molecule at the level of the entire organism as well as the at the cellular level and to gain further insight into its regulation. Specifically, we will I) Generate appropriate mouse strains lacking or overexpressing circulating Acrp30 and characterize these mice for phenotypic changes of relevant hormones and metabolites and long-term effects on adipose mass. II) Express and purify recombinant Acrp30 in a native state. III) Study the physiological changes induced by acute injection of recombinant Acrp30 in vivo in mouse and rat models IV) Define and characterize a receptor on hepatocytes and dissect the downstream signaling events that may lead to an improved response to insulin. Until now, leptin, TNFalpha and, to a lesser extent, adipsin, were representatives of a small group of proteins known to function in endocrine signaling from adipocytes. Acrp30 is a prime candidate to play an important regulatory role in whole body energy homeostasis as well, based on its adipocyte-specific expression, its structural resemblance to TNF cytokines and regulation by insulin. It is expected that these studies will contribute fundamentally toward a better understanding of the biological role of Acrp30 and broaden our perspective of adipose tissue as an endocrine, regulatory tissue.